This invention relates to slack adjusters for vehicle brakes and particularly to an improved automatic slack adjuster for cam actuated internally expanding shoe brakes of the type commonly used in large vehicles such as truck trailers.
Heavy vehicles, such as truck trailers, are typically provided with air brakes. An air brake includes an air chamber to which air is supplied during braking. When air is supplied to the chamber, a piston extends from the chamber and is connected through a linkage for rotating a cam which in turn expands brake shoes into contact with a brake drum. Then the brake pedal is released and the compressed air is vented from the chamber, the piston retracts into the air chamber and the cam is rotated in a reverse direction so that the brake shoes move clear of the brake drum. Ideally, a minimum clearance is provided between the brake shoes and the brake drum when the brakes are released. This clearance should be uniform for all brakes so that uniform braking forces are applied to the wheels on both sides of the vehicle. The clearance between the brake shoes and the brake drum should be small so that the piston rod extending from the air chamber need move only a small distance to engage the brakes. However, sufficient clearance must be provided to prevent the brake shoes from dragging on the drum, even when they are hot and experiencing thermal expansion. Any drag between the brake shoes and the drum will cause excessive wear on the shoes, over heating and excessive fuel consumption for the vehicle.
Slack adjusters are well known in the prior art for adjusting the clearance between the brake shoes and the brake drum when the brake is released. Manual adjusters are generally undesirable since they require a fairly frequent adjustment as the brake shoes become worn. If the manual adjustment is not properly performed, either an inadequate clearance is provided and the shoes will tend to drag on the brake drum or an excessive clearance is provided and the brakes will prematurely require re-adjustment.
Automatic slack adjusters for vehicle brakes are known in the prior art and are exemplified, for example, in U.S. Pat. Nos. 3,997,035, 3,997,036 and 4,057,128. These adjusters are adapted to form the link connecting the brake air chamber piston rod with the cam shaft for rotating the cam shaft in response to linear movement of the piston rod. The automatic slack adjuster includes a worm gear which is keyed or connected through splines to the cam shaft which moves the brake shoes. A worm mounted to rotate about an axis perpendicular to the cam axis engages the worm gear for establishing the angular position between a housing for the slack adjuster and the cam shaft. Various devices are provided in the prior art for rotating the worm gear to automatically change the angular position between the slack adjuster housing and the cam shaft for eliminating slack as the brake shoes become worn. However, problems have occurred with prior art slack adjusters due to poor reliability, excessive weight, complicated and expensive construction and in providing a design which accurately establishes a desired clearance between the brake shoes and the brake drum when the brakes are released. Problems also have occurred in prior art slack adjusters from both high frequency and low frequency vibration.